1. Field of the Invention
The present invention relates to a microcomputer system, and more particularly to a microcomputer system in which the power consumption can be reduced.
2. Description of the Prior Art
Referring to a block diagram in FIG. 7, a conventional microcomputer system will be explained.
Referring to the FIG. 7, numeral 1 represents a microcomputer system such as personal computer, control unit, and the like. The microcomputer system 1 comprises a central processing unit (hereinafter referred to as CPU) 2, a micro processing unit (hereinafter referred to as MPU) 5 consisted of the CPU 2 and a buffer circuit 4 which is connected thereto through an internal bus 3, a ROM (read only memory) 7 and a RAM (random access memory) 8 as external memories connected through the buffer circuit 4 and an external bus 6, peripherals including a data input/output unit (hereinafter referred to as I/O unit) 9 such as parallel port and serial port, a clock generator 12 connected to the MPU 5, and a clock 13 having counter and the like connected to the external bus 6.
The MPU 5, ROM 7, RAM 8, and I/O unit 9 are, for example, consisted of one-chip LSI, respectively. Those are supplied with power from a power source section through a power supply line PW. The CPU 2 drives the external bus 6 through the buffer circuit 4 so as to read out program stored in the ROM 7, to write data and user program into the RAM 8, or to read out the data and program written in the RAM 8. Also, the CPU 2 inputs and outputs data to and from external devices through the I/O unit 9.
The microcomputer system 1 has to read out programs and data from the ROM 7 and the RAM 8 in order to execute programs by means of the CPU 2. Therefore, an address value storing a program is firstly outputted to the ROM 7 or RAM 8 through the internal bus 3, buffer circuit 4 and the external bus 6. Then, the CPU 2 reads the program or the data stored in the ROM 7 or RAM 8 through the buffer circuit 4 and the internal bus 3. Next, the CPU 2 performs processings corresponding to the program and data. When the data is to be outputted to the ROM 7, RAM 8, and the I/O unit 9, it is outputted again through the internal bus 3, buffer circuit 4, and the external bus 6.
Since the conventional microcomputer system is constructed as the above description, the external bus 6 to which the ROM 7, RAM 8 and the I/O unit 9 are connected is to be always accessed, which consumes more power than needed. Further, since input/output of the data is always necessary for operating as a system, the power source of the peripherals (ROM 7, RAM 8, I/O unit 9) can not be turned off or the power can not be reduced, so that the power consumption of overall system can not be restrained from increasing.